We describe Arceuthobium abietinum subspecies wiensii (Wiens' dwarf mistletoe, Viscaceae), a dwarf mistletoe that severely parasitizes red fir and Brewer spruce in northwestern California and southwestern Oregon. This classification is based on morphological and host range differences between white fir dwarf mistletoe (Arceuthobium abietinum f. sp. concoloris), red fir dwarf mistletoe (Arceuthobium abietinum f. sp. magnificae), and Wien's dwarf mistletoe. Male and female plants of Wiens' dwarf mistletoe were consistently smaller than those of both white fir and red fir dwarf mistletoes, whose plants were about the same size. The shoot color of Wiens' dwarf mistletoe was frequently green-brown or red-brown, while the shoot color of white fir and red fir dwarf mistletoes was typically yellow-green or yellow. Differences in host specificity also distinguish Wiens' dwarf mistletoe from white and red fir dwarf mistletoes.